In recent years, for the preparation of polyesters such as polyethylene-terephthalate or polybutyleneterephthalate, it is the general practice to use a process of directly reacting terephthalic acid with glycols without using dimethyl-terephthalate as a starting material. This process is called a direct polymerization process. In this process, the terephthalic acid used as the starting material should have a quality as high as possible.
Terephthalic acid used for the direct polymerization has been prepared by a process in which crude terephthalic acid crystals obtained by air oxidation of a p-xylene in liquid phase have been once separated for collection, after which they are dissolved in a water solvent under high temperature and high pressure conditions and purified by hydrogenation in the presence of a palladium-carbon catalyst. However, the purified terephthalic acid obtained by the above process contains a small amount of impurities which emit fluorescence at a wavelength of 420 to 480 nm. Accordingly, the terephthalic acid was rather unsuitable as a starting material for the preparation of polyesters because color rendering (i.e. a tendency to produce a hue difference according to a light source) is one of problems to solve.
On the other hand, another process is known in which a slurry containing terephthalic acid crystals obtained by air oxidation of p-xylene in liquid-phase is subsequently subjected to post-oxidation under certain conditions to obtain terephthalic acid usable as the direct polymerization. (Japanese Patent Publication No. 47864/1987) This process is not only advantageous in that high-quality terephthalic acid is obtained in one plant, but also advantageous in that the terephthalic acid contains a reduced amount of fluorescent impurities.
However, the quality of terephthalic acid is generally expressed by alkali transmission wherein the wavelength is approximately from 340 to 400 nm from the standpoint of the sensitivity of measurement. With the above terephthalic acid, even though the alkali transmission is good, the whiteness of a polymer obtained from the terephthalic acid is not necessarily satisfactory.
Under these circumstances in the art, the present inventors have made extensive studies of a process of preparing terephthalic acid useful in the direct polymerization, in which a slurry containing terephthalic acid produced by air oxidation of p-xylene in liquid phase is subjected to post-oxidation, so as to obtain terephthalic acid of high quality. When the terephthalic acid is used to prepare polyesters, the resultant polymer has an improved degree of whiteness. As a result, it has been found that the quality of terephthalic acid should not be evaluated by alkali transmission, but should be assessed by spectral reflectance at 400 to 500 nm in order to permit a closer relation to the whiteness of a polymer obtained from the terephthalic acid. Based on the above finding, the present inventors have made further studies on the preparation of high-quality terephthalic acid. As a result, it has also been found that in order to obtain high-quality terephthalic acid useful in preparing polyesters whose whiteness is good, the spectral reflectance of terephthalic acid crystals in a slurry prior to the post-oxidation should be controlled to be within a certain range. The present invention is accomplished based on the above findings.